The present invention relates to a positioning device which utilizes signals from positioning satellites, a method of controlling a positioning device, and a recording medium.
A positioning system has been used in practice which locates the present position of a GPS receiver utilizing a satellite positioning system (SPS) such as a global positioning system (GPS).
The GPS receiver receives signals from GPS satellites and calculates the distance between each GPS satellite and the GPS receiver (hereinafter called “pseudo-range”) based on the difference between the time at which the signal is transmitted from each GPS satellite and the time at which the signal reaches the GPS receiver (hereinafter called “delay time”). The GPS receiver calculates (locates) the present position using the pseudo-range and satellite orbital information of each GPS satellite contained in the signal received from each GPS satellite.
When the GPS receiver can receive signals from four or more GPS satellites, the GPS receiver can perform three-dimensional positioning which calculates the latitude, the longitude, and the height of the present position.
When the GPS receiver can receive signals from three GPS satellites, the GPS receiver can perform two-dimensional positioning which calculates the latitude and the longitude of the present position. For example, the GPS receiver regards the center of the earth as one GPS satellite, and uses the distance between the center of the earth and the present position as the pseudo-range. The GPS receiver then calculates (locates) the position in the same manner as in three-dimensional positioning. Therefore, the GPS receiver must hold the height information of the present position in advance when performing two-dimensional positioning.
A two-dimensional positioning technology using the height acquired from map data (e.g. JP-A-2002-341012) and a two-dimensional positioning technology using the height calculated during the preceding positioning using a set of GPS satellites with the minimum vertical dilution of precision (VDOP) or the height calculated during the preceding three-dimensional positioning (e.g. JP-B-6-75103) have been proposed.
However, the load imposed on storage means is increased when holding the map data. Moreover, the height information obtained from the map data may have a large error.
When using the height calculated during the preceding positioning using a set of GPS satellites with the minimum VDOP, the height may have a large error due to poor positioning conditions (e.g. environment in which the signal strength is weak or environment in which multipaths occur to a large extent) during the preceding positioning. When using the height calculated during the preceding three-dimensional positioning, the height may also have a large error due to poor positioning conditions during the preceding positioning
The applicant of the invention has filed a patent application regarding technology of updating the height information taking the conditions during the preceding positioning into consideration (Japanese Patent Application No. 2005-151048 (published as JP-A-2006-329705)). However, since this technology updates the height information when the three-dimensional height information has specific reliability, the height information cannot be updated when the three-dimensional height information has low reliability (e.g. multipath environment). This poses a problem in which the height information may not be promptly updated.